2 Answers
2

Hard drives tend to have boot priority ahead of USB drives. If so it is unlikely, the system will attempt to boot the USB device.

Try installing grub2 on the hard disk. You should either create a small boot partition on the disk, or configure Grub to boot the USB device. You may have to adjust the device id to reflect reordering the BIOS does when the disk is installed at boot time.

It sounds like the HDD has higher boot priority than USB. This is a good security default, protecting against both deliberate attacks via USB and a USB drive boot virus.

You have a couple options:

Go in to your BIOS settings and change the boot order. Obviously you'll need a console of some kind. Some server hardware supports a remote console, on the serial port or the LAN port, or via an out-of-band management card. If you don't have that, you'll need a crash cart.

install a boot loader on the HDD and use that to boot the USB drive. If that's OK with you, then why not install the OS on the HDD and skip the USB drive?